Individuals with IDD have obesity rates that exceed those in the general population, consume energy dense diets, and perform very little physical activity (PA). Overweight and obesity are independent risk factors for chronic disease such as cancer, diabetes, hypertension, and cardiovascular disease in both the general population and those with IDD. However to date, limited research on weight management for individuals with IDD has been completed. We have recently initiated a 6 month pilot investigation on weight loss in 73 individuals with IDD and 66 completed the 6 months. We modified the Stop Light Diet (SLDm) to include reduced energy pre-packaged meals (PM) to reduce energy and fat content, and encouraged consumption of low energy shakes and 35 fruits and vegetables (F/V) per week. SLDm resulted in an average 6.1 percent reduction in weight at 6 months and increased F/V consumption. We continue to follow the 66 participants for an additional 6 months and weight loss at 9 and 12 months is 8.2 percent and 9.1 percent from baseline. Although our pilot results are encouraging, this study was conducted in a small sample with no comparison group, and funding only allowed for measurement of weight during follow up. Therefore, we propose to conduct an adequately powered randomized trial to evaluate weight loss and weight maintenance. Social Cognitive Theory (SCT) guides the interventions using both sides of the energy balance equation (i.e. energy intake and energy expenditure) in overweight and obese individuals with IDD. We will compare participants who use the SLDm with participants who use a recommended care diet (RC). Following a 6 month period of reduced energy intake (weight loss), both groups will be placed on a diet with sufficient energy to maintain weight (i.e. weight maintenance) for and additional 12 months. Both groups will participate in a PA program that has been shown to improve metabolic risk factors such as blood pressure, triglycerides, and cardiovascular fitness, and has provided modest weight loss in gymnasium and laboratory settings. In the current proposal the PA program will be modified for home use and conducted for the entire 18 month trial. We expect the SLDm group to have greater weight loss at 6 months and less weight regain from 6 to 18 months compared to RC. We have proposed mediation analysis for components of SCT for both participant and care givers to discover what components of the intervention affected weight change. We will conduct extensive process evaluation will collect quantitative and qualitative data to assess how well the programs were implemented and accepted by participants.